Three series of experiments on the stimulus properties of drugs are proposed: The first project will develop improved preclinical methods for drug categoization based on drug discrimination procedures. It is well known after drug vs no drug discrimination training, rats can indicate whether a novel drug belongs to the same pharmacological class as the training compound. Several new training procedures will be used in this project, and we will measure the selectivity of drug classification achieved by each method. It is planned to develop methods that can split the members of a pharmacological class into several subgroups, and specify the relationship of new drugs to preexisting members of the class. The second project will localize the brain mechanisms responsible for state dependent learning (SDL). Rats will be trained to discriminate the presence of drugs injected into individual brain structures in order to identify the sites responsible for the drugs' discriminable effects. Identification of the brain structures where locally applied drugs produce SDL might identify the mechanisms that mediate SDL. The third project will investigate whether drugs can act as conditioned stimuli (CSs), and acquire conditioned responses (CRs) that they did not initially produce. Such CRs would be new (temporary) drug effects. Two prior experiments have shown such CRs to drug CSs. We will conduct a series of experiments to determine what types of conditioning procedures can attach new CRs to drug, how prolonged the conditioning procedures have to be, and how permanent or fragile are the CRs so established. The results will indicate the generality of CRs to drug, and the likelihood that such CRs can signficantly alter the efficacy of drugs during clinical use.